Pronunciation only becomes a problem when it causes a breakdown in communication between you and the people you’re talking with. Perfecting your English pronunciation is about making yourself understood.Well, I don't quite agree because incomprensibility is not the only problem; aesthetics is another one. If you as a non-native speaker aim high, you want your accent to sound pleasant; the listener should not be distracted from the contents because you frequently replace
- the dental fricatives by /s, z/ or by
- /v/ by /w/
- word-final voiced obstruents by their voiceless counterparts;
- clear l throughout
So, yes, a bad pronunciation may lead to incomprehensible speech, but it may also distract the listener.